The present invention relates to a power factor improvement circuit.
As shown in FIG. 1, in a conventional power factor improvement circuit 100 that corresponds to a boosting (step-up) type power factor improvement circuit, a bridge circuit 101 converts an AC (alternating current) input voltage into a DC (direct current) voltage. In the conventional power factor improvement circuit explained above, efficiency has been improved by technologies relating to software switching and synchronous rectification. However, in a present power factor improvement circuit that performs with high efficiency, a loss in the bridge circuit has been accounting for a large percentage among losses of main components. Accordingly, various circuit configurations have been proposed in order to reduce or eliminate the loss in the bridge circuit.
In a conventional power factor improvement circuit 200 shown in FIG. 2, during a positive period of an input alternating current, the alternating current flows through a diode (rectifying element) 201 so that a switching element 203 performs a switching operation. Further, during a negative period of the input alternating current, the alternating current flows through a diode 202 so that a switching element 204 performs a switching operation. In the circuit configuration explained above, because the number of diodes that are included in the current flow route is small, efficiency of a circuit is relatively high. However, this circuit configuration still has a problem in which common-mode noise is large.
In a conventional power factor improvement circuit 300 shown in FIG. 3, during a positive period of the input alternating current, the alternating current flows through a parasitic diode of a switching element 301 so that a switching element 302 performs a switching operation. Further, during a negative period of the input alternating current, the alternating current flows through a parasitic diode of the switching 302 so that the switching element 301 performs a switching operation. However, because of a recovery problem of the parasitic diodes of the switching elements 301 and 302, the switching operation is limited to a discontinuous current mode.
In a conventional power factor improvement circuit 400 shown in FIG. 4, during a positive period of the input alternating current, a switching element 401 performs a switching operation. Further, during a negative period of the input alternating current, a switching element 402 performs a switching operation. In the circuit configuration explained above, diodes 403, 404 and inductor elements 405, 406 can solve the common-mode noise problem. However, because two inductor elements 405, 406 are needed, the size of the circuit becomes large.
In the same way as FIG. 4, in a conventional power factor improvement circuit 500 shown in FIG. 5, during a positive period of the input alternating current, a switching element 501 performs a switching operation. Further, during a negative period of the input alternating current, a switching element 502 performs a switching operation. In the circuit configuration explained above, diodes 503, 504 and a transformer 505 can solve the common-mode noise problem. However, because the transformer 505 (a mutual inductor) is needed, the size of the circuit becomes large.
In a conventional power factor improvement circuit 600 shown in FIG. 6A, during a positive period of the input alternating current, a switching element 601 performs a switching operation. Further, during a negative period of the input alternating current, a switching element 602 performs a switching operation. As a result, a loss in a bridge circuit can be reduced. Here, a driving ground node 603 is connected to a ground potential for a driving circuit of the switching elements 601, 602. When a terminal L is positive, the driving ground node 603 is stable without swinging with respect to a terminal N that is in a stable voltage state. In contrast, when the terminal N is positive, the driving ground node 603 is connected to an inductor through a parasitic diode of the switching element 601. In this case, because the switching element 602 performs the switching operation, a voltage potential of the driving ground node 603 swings (hopping) with a switching frequency (a high frequency) at a half of a cycle with respect to the terminal N as shown in FIG. 6B. Therefore, the common-mode noise can easily occur. In addition, because the driving ground node 603 is not stable, the common-mode noise due to swinging of the driving ground node 603 is converted to a system common-mode noise by a power supply system (the driving circuit). Thus, an anti-coherence (an anti-noise) property of the driving circuit is highly required.